Scientists study the behavior of gases extensively. One topic of interest is how a gas cools as it expands. To accurately measure this, it is best to do it under adiabatic conditions. Theoretically adiabatic conditions would mean that absolutely no heat could be exchanged between the system (the gas) and the surrounding environment. While this isn’t possible, the temperature of a gas expanding under well-insulated conditions will suffice.
Steps
Method 1
Method 1 of 3:
Mimicking Adiabatic Conditions
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Connect 2 insulated containers. One container will be used to house the gas before the expansion. The second container will be empty and provide additional volume for the gas to occupy as it expands. Use a valved hose or pipe to connect these containers so that the contents of the connected containers can be closed off from one another. [1] X Research source
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Pressurize the first container. Check the valve between the containers to confirm that it is closed. Use a cylinder of compressed air to force air into the first container. Pressurize the container to approximately 1.5 atm. [2] X Research source
- Use a barometer to check the pressure.
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Evacuate the second container. Ensure that the valve between the containers remains closed. Use a vacuum pump to evacuate gas from the second container. It will not be possible to remove all gas from the container, but you should make sure that the container is approximately 0.1 atm. [3] X Research source
- A barometer can be used to determine the negative pressure (vacuum).
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Method 2
Method 2 of 3:
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Take initial temperature readings of the gas. After you have pressurized the first container, let the gas sit undisturbed for approximately 5 minutes. This should be sufficient time for the temperature to equilibrate and provide an accurate reading. Take a temperature reading of the gas inside the container and record it as your initial temperature. [4] X Research source
- Using a container with a lid or stopcock (a valve that regulates the flow of the gas) is ideal. This way you can insert the thermometer into the stopcock to measure the temperature without letting gas escape.
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Allow the gas to expand into both containers. Open the valve. The gas will move from the high pressure container to the low pressure container until it reaches equilibrium. This is known as expansion.
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Take a final temperature reading. Once the gas has expanded, take a second temperature reading. Record this reading as your final temperature. Notice that this temperature should be lower than your initial temperature. [5] X Research sourceAdvertisement
Method 3
Method 3 of 3:
Understanding the Results
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Consider the ideal gas law. The ideal gas law describes the behavior of a hypothetical gas in which molecules do not interact with each other, except for totally elastic collisions. Elastic collisions will result in no net energy change. According to this law, if volume increases and pressure is held constant, temperature would increase. Thus, it is clear that real gases do not behave in an “ideal” way. The equation describing this behavior is PV=nR: [6] X Research source
- P = Pressure
- V = Volume
- n = Moles of Gas
- R = Ideal Gas Constant
- T = Temperature
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Realize that gas molecules interact with each other. The primary difference between ideal and real gases is that real gases have molecules that interact in inelastic ways. That is to say, the energetic state of the molecules changes based on how they interact with each other. [7] X Research source
- This means that heat will rise in a smaller volume of gas (due to more collisions between molecules) and will fall when the same amount of gas occupies a larger volume.
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Keep in mind that a truly adiabatic system does not exist. It is important to realize that adiabatic conditions are theoretical. There is no such thing as a perfectly insulated system, and there will be some heat exchange with the outside universe. That said, if measurements are taken quickly enough, that exchange can be negligible for this experiment. [8] X Research sourceAdvertisement
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Tips
- Enroll in a class on thermodynamics to gain a deeper understanding of this phenomenon.Thanks
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Warnings
- Wear goggles when working with pressurized systems.Thanks
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Things You’ll Need
- Two insulated containers
- Hose
- Valve
- Cylinder of compressed air
- Vacuum pump
- Thermometer
References
- ↑ http://www.eoht.info/page/Joule%E2%80%99s+second+law
- ↑ http://www.eoht.info/page/Joule%E2%80%99s+second+law
- ↑ http://www.eoht.info/page/Joule%E2%80%99s+second+law
- ↑ http://www.eoht.info/page/Joule%E2%80%99s+second+law
- ↑ http://www.eoht.info/page/Joule%E2%80%99s+second+law
- ↑ http://h2g2.com/entry/A1066259
- ↑ http://h2g2.com/entry/A1066259
- ↑ http://www.chem1.com/acad/webtext/energetics/CE-2.html
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